The Xindi-Reptilians had yellow to brown-colored scales with multiple spikes coming out of their heads. They had yellow eyes with slitted vertical pupils, and were cold blooded. Like other Xindi, they had characteristic ridges on their cheek bones. They also were stronger and had greater stamina than humans.
Reptilians were perceived as being untrustworthy and impatient by the other Xindi species. (ENT: "The Shipment") A common Reptilian saying was "Patience is for the dead." It was said that "Dealing with Reptilians is like bargaining with the sun. You make no progress, and you come away burned." Nevertheless, the Reptilians were the first choice for when force was needed. (ENT: "Azati Prime")
With their easily provoked and opinionated nature, Xindi-Insectoids often sided with the Reptilians on certain issues. They sometimes even shared ships with each other. (ENT: "Rajiin") Reptilian soldiers were surgically implanted with a "suicide gland" that secreted a neurotoxin when they were captured.
The Reptilians were the species of Xindi that seemed to be most interested in destroying the planet Earth. The Reptilians lived out a militaristic lifestyle and hoped to ultimately rule over all the other species of Xindi. (ENT: "Rajiin")
Reptilians preferred to be on or near the ground as opposed to up in the air.
|Aquatic • Arboreal • Avian • Insectoid • Primate • Reptilian|
Representing each of the Xindi-Reptilians' faces involved the use of a delicate, full-facial latex prosthetic which was extensively airbrushed to create skin texture, shadows and definition. The small tusks protruding from near the mouth and the black spikes extending from the head were made of resin.  Makeup supervisor Michael Westmore once referred to the latter protrusions as "porcupine quills," presumably because his idea for the spikes was influenced by the coat of a porcupine. 
The Reptilians' uniforms were reused Reman costumes from Star Trek Nemesis. However, broad "cages" of wire armor were added for the Reptilians' reuse of the costumes. Those components were extremely delicate, so a great deal of care had to be taken to avoid damaging them. ("Countdown" text commentary, ENT Season 3 DVD)
As well as being portrayed by live actors and stuntmen, Xindi-Reptilians were also depicted with a mannequin (in the case of the Xindi-Reptilian corpse in "The Expanse") and computer-generated figures (such as in the first shot of the Xindi weapon's interior, in "Countdown"). (; "Countdown" text commentary, ENT Season 3 DVD)
The makeup for the Xindi-Reptilians influenced the CGI design of the Xindi-Insectoids and -Aquatics. (Star Trek: Communicator issue 149, pp. 51 & 52) Also, with the Reptilians having been designed before the Xindi-Primates, the fact that the common Xindi trait of cheek ridges had been selected as part of the Reptilian design made designing the Primates an easy task. (Star Trek: Communicator issue 149, p. 56)
In a 2010 interview, Michael Westmore cited the Xindi-Reptilians as one of "two things I loved doing on Enterprise." He went on to refer to the Reptilians as "fascinating" and further remarked, "The heads, with the porcupine quills coming out of them, were gorgeous."